Controller with fractional position algorithm

ABSTRACT

One illustrative embodiment of the present invention pertains to a device. The device is configured to evaluate a Gray code value and to provide compensation for error in the Gray code value. Another illustrative embodiment of the present invention pertains to a method. The method includes comparing Gray code information with servo information. The method also includes evaluating error in the Gray code information based on the comparison between the Gray code information with servo information. Another illustrative embodiment of the present invention pertains to a data storage system. The data storage system includes a data storage medium, a head, and a controller. The head is controllably positionable relative to the data storage medium. The controller includes a means for sensing position information from the head. The controller also includes a means for compensating for error in the position information, to position the head relative to the data storage medium. Embodiments of the present invention provide unforeseen and inventive advantages over conventional data storage systems, including by assuring superior position evaluation and control of a read head relative to a data storage medium.

FIELD OF THE INVENTION

The present invention relates generally to data storage systems, and inparticular to a controller with a fractional position algorithm.

BACKGROUND OF THE INVENTION

Data storage systems have tended to be made ever smaller, yet with evergreater storage capacity, as technology has advanced. Such data storagesystems are usefully applied in a wide variety of settings includingcomputers, networks, digital music players, PDAs, digital still camerasand video cameras, and external computer memory, among a wide variety ofother possible examples. One limit on the performance of a data storagesystem is the accuracy with which the system can evaluate and controlthe position of a read/write head or other form of read head relative tothe positions of data within the system. Providing data storagetechnology with optimum performance in current applications posesconsiderable technical challenges. However, there remains a persistentneed for providing data storage systems that are ever smaller, yet withever greater storage capacity and superior performance characteristics.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide unforeseen and inventiveadvantages over conventional data storage systems, including by assuringsuperior control of a read/write head, or other form of read head,relative to a data storage medium, as an illustrative example.

One illustrative embodiment of the present invention pertains to adevice. The device is configured to evaluate a Gray code value and toprovide compensation for error in the Gray code value.

Another illustrative embodiment of the present invention pertains to amethod. The method includes comparing Gray code information with servoinformation. The method also includes evaluating error in the Gray codeinformation based on the comparison between the Gray code informationwith servo information.

Another illustrative embodiment of the present invention pertains to adata storage system. The data storage system includes a data storagemedium, a head, and a controller. The head is controllably positionablerelative to the data storage medium. The controller includes a means forsensing position information from the head. The controller also includesa means for compensating for error in the position information, toposition the head relative to the data storage medium.

Other features and benefits that characterize various embodiments of thepresent invention will be apparent to those skilled in the relevant artfrom the description herein and the associated drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a data storage system, according to oneillustrative embodiment.

FIG. 2 is a schematic of a representative section of a data storagemedium, according to an illustrative embodiment.

FIG. 3 is a flowchart diagram of a fractional position algorithm for acontroller of a data storage system, according to an illustrativeembodiment.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Embodiments of the present invention provide unforeseen and inventiveadvantages over conventional data storage systems, including by assuringsuperior control of a read/write head, or other form of read head,relative to a data storage medium, as an illustrative example. Forexample, such superior control may be provided by a controller or otherdevice that is configured to receive position information from a readhead and to use the position information to evaluate a track offsetposition of the read head, including by compensating for any error in aGray code comprised in the position information. Such a controller orother device may be incorporated in a data storage system. Someillustrative embodiments are described herein. Although the examplesbelow show more than enough detail to allow those skilled in the art topractice the present invention, subject matter regarded as the inventionis broader than any single example below.

To avoid needless distractions from the essence of the presentinvention, like-numbered reference numerals appearing in a later figuregenerally refer to the same elements as those in an earlier figure.Also, numerous aspects of basic engineering and of positioningtechnologies that are not a part of the present invention (or are wellknown in the art) are omitted for brevity. For example, this documentdoes not articulate detailed and diverse methods for writing a servosector. Neither does it include implementation decisions such as whatthe bit density will be on each track. Specific techniques forconstructing disc stacks are likewise omitted, typically being a matterof design choice to those of ordinary skill in that field of technology.

FIG. 1 depicts data storage system 105, which may incorporate acontroller or other device with a fractional position algorithm,according to one illustrative embodiment. FIG. 1 depicts is an exploded,perspective view of a data storage system 205, illustratively embodiedas a disc drive in this embodiment, which includes disc 100, accordingto an illustrative embodiment.

Disc drive 105 is one example from a variety of data storage systems towhich various embodiments are applicable. Disc drive 105 includes ahousing with a deck 112 and a top cover (not shown). Disc drive 105 alsoincludes a disc pack 114 comprising representative disc 100 and severalother, similar discs. Disc pack 114 is rotatably mounted on deck 112 ona spindle motor (not shown) by a disc clamp 116. Disc pack 114 includesa plurality of individual discs which are mounted for co-rotation aboutcentral axis 118. Each disc surface has an associated slider, such asrepresentative slider 120, which is mounted to disc drive 105 andcarries a data interface head such as a read/write head or other form ofread head (not separately shown), with read and/or write function, onslider 120 for communication with the respective disc surface, such asrepresentative disc surface 128, in this illustrative embodiment. Theread/write head on head-bearing slider 120 is capable of reading datafrom and writing data to disc surface 128, in this illustrativeembodiment. The data is generally written along a series of concentricor spiral data tracks written on media surface 128, for example. Theread/write heads may be of any type known in the art or yet to bedeveloped, including magnetic, magnetoresistive, giant magnetoresistive(GMR), optical, and so forth, in various embodiments. In differentembodiments, a wide variety of numbers of discs, read/write heads, andhead-bearing sliders, may occur.

In FIG. 1, representative slider 120 is supported by suspension 110which is rotatably mounted on deck 112. More particularly, suspension110 is rotatably mounted on actuator 126, included on deck 112, and isthereby disposed on deck 112 in a controllably moveable way. Suspension110 supplies a pre-load force to slider 120 which is substantiallynormal to opposing disc surface 128. The pre-load force counteracts anaerodynamic lifting force developed between slider 120 and disc surface128 during the rotation of disc pack 114. Each disc surface is likewiseinterfaced by a similarly disposed slider (not shown). Actuator 126 is arotary moving coil actuator and includes a voice coil motor, showngenerally at 130, in this illustrative embodiment. Voice coil motor 130rotates actuator 126 about pivot shaft 132 to position slider 120 overan intended data track (not shown in FIG. 1) along a slider range 134between a disc inner diameter 136 and a disc outer diameter 138. Otherelements may occur in alternative embodiments, such as an actuator thatpositions the read/write head through linear extension and retraction,for example.

Voice coil motor 130 operates under control of internal circuitry 139.Internal circuitry 139 may include software or firmware for controllingthe operation of data storage system 105, for example. Such software orfirmware may include computer-executable instructions included on acomputer-readable medium, along with a processor configured to executethose instructions. The computer-executable instructions may alsoconfigure the processor to perform further tasks, such as receiveposition information, produce evaluated values based on that informationsuch as a predicted position of a read head, and send controlinstructions based on those evaluated values, for example. Internalcircuitry 139 may be considered a controller, or alternatively a part ofa controller together with other elements of data storage system 105,for the operation of the sliders including representative slider 120,along with the read/write heads associated with the sliders. Internalcircuitry 139 may therefore include algorithms for reading and writingdata from and to the media surfaces such as representative media surface128, and for functions involved in supporting such reading and writingof data, such as controllably positioning the read/write heads relativeto the media surfaces, and relative to tracks on the media surfaces, inan illustrative embodiment.

FIG. 2 depicts a schematic of a representative small portion 200 of amedia surface, such as media surface 128, that includes a radiallyrepeating servo burst pattern, as will be well understood by those whoare skilled in the relevant art. Media surface portion 200 includestrack centerlines 211 and 213. These are representative of concentric orspiral tracks disposed on media surface 128, that are largely availablefor the storage of user data, and also include operating informationsuch as servo and Gray code information, for example. In this view, thecenter of the media surface, which may coincide with its center ofrotation within a data storage system, lies off to one side in adirection perpendicular to track centerlines 211 and 213. A mediasurface typically contains a great many of these tracks. For example, inone illustrative embodiment, a media surface may include in theneighborhood of 100,000 tracks per inch, and may for example have adiameter of 3.5 inches, 2.5 inches, 1 inch, or a fraction of an inch.Many other track densities and media surface sizes, both higher andlower than these examples, may occur in various embodiments.

Media surface portion 200 includes Gray code sector 201, zone sector203, and a position error signal (PES) sector bounded by PES boundary205 and PES boundary 209. Gray code sector 201 and PES sector 205-209contribute to position information that is read by a read/write head oranother type of read head, and used to evaluate a track offset positionof the read head—that is, a position by which the head is offset from atrack centerline such as centerlines 211, 213. This track offsetposition is useful, for instance, for guiding the head back toward thedesired track centerline, or otherwise compensating for the head'sperformance issues from being away from the track centerline, forexample.

PES sector 205-209 includes servo burst information including servoburst component N, indicated with function 221, and servo burstcomponent Q, indicated with function 223. N and Q are two servo burstcomponents, incorporating servo burst amplitude values such as might bemeasured by a read head, and useful for deriving a position error signal(PES). In this illustrative embodiment, the absolute value of Q reachesits maxima at the centerlines 211, 213 of the tracks, while the absolutevalue of N reaches its maxima at the Gray code transitions 215, 217, 219halfway between each pair of adjacent track centerlines. A number ofdifferent mechanisms for deploying the N and Q components may occur indifferent embodiments. For example, in one illustrative embodiment, Nand Q are the superimposed sums of four servo bursts A, B, C and D, suchthat N=A-B and Q=C-D, where A and B alternate with each other, and C andD alternate with each other, while the A/B cycle is offset by half aservo burst from the C/D cycle. This is one of several examples that areknown or possible that may be used to generate an N and Q pattern suchas illustratively depicted in FIG. 2.

A data storage system may use a fractional position algorithm that usesposition information, such as Gray code, N and Q values, as input togenerate a PES and/or to evaluate a track offset position of the readhead, for example. Some earlier systems used a fractional positionalgorithm that could handle an indeterminate Gray code bit within plusor minus 25% of the Gray code transition, as seen for example in FIG. 2with the “25% off” lines at each halfway mark between representativecenterlines 211, 213 and representative Gray code transitions 215, 217and 219. It was found in such systems that an indeterminate Gray codeleast significant bit was exceeding the plus or minus 25% limit of thefractional position algorithm. This sometimes caused a “spike” in thePES, which involved a one sample position error excursion outside awrite fault threshold, which would disable the write function to preventwriting onto an incorrect position. It was also found, as a largerconcern, that the excessive indeterminate Gray code bit was sometimesnot causing a position error signal outside the write fault threshold,and thereby allowing an apparently valid but actually incorrectevaluation of head position, which would allow the head to write ontothe media surface even though it was at an incorrect position. Thiserroneous behavior is resolved in embodiments disclosed herein, whichcan compensate for an indeterminate Gray code bit occurring within plusor minus 50% of a Gray code transition—in other words, right up to thetrack centerlines (e.g. 211, 213), and therefore anywhere on the mediasurface. Illustrative embodiments disclosed herein are thereforeconfigured to evaluate a track offset position of the read head,including by compensating for any error in a Gray code comprised in theposition information, no matter where the Gray code occurs relative tothe track centerlines (e.g. 211, 213) and the Gray code transitions(e.g. 215, 217, 219).

FIG. 3 is a flowchart diagram of a fractional position algorithm 300 fora controller of a data storage system, according to an illustrativeembodiment. Fractional position algorithm 300 is an illustrativeembodiment of an algorithm configured to receive position information,including a value for N, a value for Q, and a Gray code value, and topredict a position of a read head as a function of the positioninformation. Algorithm 300 evaluates the values of N and Q prior toevaluating the Gray code value. Fractional position algorithm 300 mayconsist of computer-executable instructions included in acomputer-readable medium such as internal circuitry 139, for example,which can be executed by a processor, also included in internalcircuitry 139, that controls data storage system 105, to configure theprocessor to perform functions such as receive the position informationthat serves as input for the algorithm, and to use the output of thealgorithm to predict a position of a read head as a function of theposition information, in one illustrative embodiment.

As seen in FIG. 3, the algorithm 300 first performs step 301 to evaluatewhether the received Q value is less than zero. If yes, it proceeds tostep 311; if no, it proceeds to step 313. Next, algorithm 300 performseither step 311 or step 313, both of which are to evaluate whether thereceived N value is greater than zero. From step 311, if yes, then itproceeds to step 321, and if no, it proceeds to step 323. From step 313,if yes, then it proceeds to step 327, and if no, it proceeds to step325. As the third step, regardless of which of the four paths algorithm300 has followed by now, it next evaluates whether the Gray code valueis even or odd. Specifically, if proceeding from step 321 or step 323,algorithm 300 evaluates whether the Gray code value is even, and ifproceeding from step 327 or step 325, algorithm 300 evaluates whetherthe Gray code value is odd. Algorithm 300 evaluates the Gray code valuein these specific forms because the expected answer in each case is yes,assuming the Gray code value is correct for the position correspondingto the evaluated values of Q and N. As can be seen with reference againto FIG. 2, for step 321 to be reached and therefore for the Q line 223to be less than zero and the N line 221 to be greater than zero, theposition should be in zones 1 or 2, and the Gray code value should beeven. For step 323, i.e. Q less than zero and N less than zero, theposition should be in zones 3 or 4, and the Gray code value should beeven. For step 325, i.e. Q greater than zero and N less than zero, theposition should be in zones 5 or 6, and the Gray code value should beodd. For step 327, i.e. Q greater than zero and N greater than zero, theposition should be in zones 7 or 8, and the Gray code value should beodd.

Therefore, at steps 321, 323, 325, and 327, the expected answer is yes,and an answer of no indicates an error in the Gray code value, such thatthe Gray code least significant bit does not match with the zoneindicated from the Q and N values. Accordingly, for each of steps 321,323, 325, and 327, if a yes answer is received, it proceeds to the nextevaluation step, i.e. steps 341, 343, 345, or 347, respectively; but ifa no answer is received, algorithm 300 detours first to a positioncompensation step, i.e. steps 331, 333, 335, or 337, respectively. Steps331, 333, 335, or 337 introduce a correction factor x of either +1 or −1that will be used to compensate for the error in the Gray code value, inevaluating the track offset position of the read/write head or otherform of read head. Then, whether or not the appropriate one of steps331, 333, 335, or 337 has been triggered, algorithm 300 proceeds to theappropriate next evaluation step, i.e. step 341, 343, 345, or 347,respectively.

Steps 341, 343, 345, and 347 are intended to finish the task ofevaluating which of the eight zones depicted in FIG. 2 corresponds tothe track offset position of the read head, such as a read/write head insome embodiments. The specific evaluation performed depends on whichstep is performed. Step 341, which is performed if the Q value is lessthan zero and the N value is greater than zero, involves evaluatingwhether the N value is greater than negative one times the Q value. Step343, which is performed if the Q value is less than zero and the N valueis not greater than zero, involves evaluating whether the N value isgreater than the Q value. Step 345, which is performed if the Q value isnot less than zero and the N value is not greater than zero, involvesevaluating whether the N value is less than negative one times the Qvalue. And step 347, which is performed if the Q value is not less thanzero and the N value is greater than zero, involves evaluating whetherthe N value is less than the Q value. These are all specific todetermining which particular one of the eight zones corresponds to thefractional track offset position of the read head, depending on theinformation already gained. Each one is sufficient to distinguishbetween which of the two remaining possible zones corresponds to the Qand N values already evaluated. Steps 341 and 345 also lead to anadditional compensation step, i.e. steps 351 or 355, respectively, ifthe evaluation results in a yes answer, while steps 343 and 347 alsolead to an additional compensation step, i.e. steps 353 or 357,respectively, if the evaluation results in a no answer. These additionalcompensation steps correspond to an evaluated position that is withinplus or minus 25% of one of the Gray code transitions (e.g. 215, 217,219), and they further modify the correction factor x by either +0.5 or−0.5.

In those cases where the Gray code least significant bit does not match,in the zones that are more than 25% of the way from a Gray codetransition—i.e. zones 2, 3, 6 and 7—the final result of the algorithmincludes a value for x that is equal to 1 or −1, which will indicate aclear error in the position signal. If an incorrect adjustment is madeto the output, a large position error signal will be generated,triggering the generation of a substitute value. This system thereforeensures that an erroneous Gray code least significant bit will alwayscause a position error signal outside the write threshold, andpreventing the head from writing onto the media surface when it is in anincorrect position.

Algorithm 300 thereby configures the controller or data storage systemrunning it to compensate for an indeterminate least significant bit inthe Gray code value in a zone within plus or minus 25% of a trackcenterline. The end result for the value of the correction factor x issummarized in Table 1, below. TABLE 1 ZONE Position InformationGenerated X Value 1 Q < 0, N > 0, GC odd, N > −Q +0.5 2 Q < 0, N > 0, GCodd, N < −Q +1 1 Q < 0, N > 0, GC even, N > −Q −0.5 2 Q < 0, N > 0, GCeven, N < −Q +0 3 Q < 0, N < 0, GC odd, N > Q −1 4 Q < 0, N < 0, GC odd,N < Q −0.5 3 Q < 0, N < 0, GC even, N > Q +0 4 Q < 0, N < 0, GC even, N< Q +0.5 5 Q > 0, N < 0, GC even, N < −Q +0.5 6 Q > 0, N < 0, GC even,N > −Q +1 5 Q > 0, N < 0, GC odd, N < −Q −0.5 6 Q > 0, N < 0, GC odd,N > −Q +0 7 Q > 0, N > 0, GC even, N < Q −1 8 Q > 0, N > 0, GC even, N >Q −0.5 7 Q > 0, N > 0, GC odd, N < Q +0 8 Q > 0, N > 0, GC odd, N > Q+0.5

Algorithm 300 thereby configures the controller included in a datastorage system to use position information received from a read head toevaluate a track offset position of the read head, and within thatprocess, to compensate for any error that might occur in a Gray codevalue contained within the position information, and to generate aposition error signal for the read head that includes any suchcompensation for an erroneous Gray code value. Algorithm 300 alsothereby ensures that if an adjustment made in evaluating the trackoffset position is incorrect, a resulting position error signal will belarger than a validity threshold, i.e. the write fault threshold,corresponding to the position being obviously invalid, in which case anestimated value will be provided. The estimated value may be generatedby additional algorithms or systems, such as those currently known tothose in the art, or other systems, for example.

The present invention therefore includes unexpected and novel advantagesas detailed herein and as can be further appreciated from the claims,figures, and description by those skilled in the art. Although some ofthe embodiments are described in reference to a controller and/or a datastorage system, the present invention has various other embodiments withapplication to other devices, systems and applications in which positioninformation is used to evaluate a position.

It is to be understood that even though numerous characteristics andadvantages of various illustrative embodiments of the invention havebeen set forth in the foregoing description, together with details ofthe structure and function of various embodiments of the invention, thisdisclosure is illustrative only, and changes may be made in detail,especially in matters of structure and arrangement of parts within theprinciples of the present invention, to the full extent indicated by thebroad, general meaning of the terms in which the appended claims areexpressed. It will be appreciated by those skilled in the art that theteachings of the present invention can be applied to a family ofsystems, devices, and means encompassed by and equivalent to theexamples of embodiments described, without departing from the scope andspirit of the present invention. Further, still other applications forvarious embodiments, including embodiments pertaining to a variety ofother servo burst values, data storage systems, and relatedtechnologies, are envisioned within the scope of the present inventionas claimed herein.

1. A device configured to evaluate a Gray code value and to providecompensation for error in the Gray code value.
 2. The device of claim 1,further configured to generate a signal for positioning a head relativeto a data medium, wherein the signal comprises the compensation forerror in the Gray code value.
 3. The device of claim 2, furtherconfigured to adjust the signal to be greater than a position errorsignal threshold, responsively to the error in the Gray code value. 4.The device of claim 3, further configured to adjust the signalresponsively to the error in the Gray code value if a least significantbit comprised in the Gray code value is indeterminate in a zone withinplus or minus 25% of a track centerline of the data medium.
 5. Thedevice of claim 3, further configured to provide an estimated positionerror signal, if the signal is adjusted to be greater than a positionerror signal threshold.
 6. The device of claim 1, further configured toreceive the Gray code value with position information from a headinteracting with a data medium.
 7. The device of claim 6, furtherconfigured to evaluate a track offset position of the head relative to aGray code transition on the data medium.
 8. The device of claim 6,further configured to evaluate servo burst amplitudes comprised in theposition information, and wherein evaluating error in the Gray codevalue comprises comparing the Gray code value to the servo burstamplitudes.
 9. The device of claim 8, further configured to evaluatevalues of N and Q from among the servo burst amplitudes beforeevaluating the Gray code value.
 10. The device of claim 9, furtherconfigured to compare the Gray code value to the values of N and Q. 11.The device of claim 10, further configured to compare the values of Nand Q with each other, after evaluating the Gray code value.
 12. Thedevice of claim 9, further configured to evaluate the track offsetposition of the read head by: first, evaluating whether a Q value isless than zero; second, evaluating whether an N value is greater thanzero; and third, evaluating whether the Gray code value is even or odd.13. The device of claim 12, further configured to evaluate the trackoffset position of the read head by: fourth, if the Q value is less thanzero and the N value is greater than zero, then evaluating whether the Nvalue is greater than negative one times the Q value; if the Q value isless than zero and the N value is not greater than zero, then evaluatingwhether the N value is greater than the Q value; if the Q value is notless than zero and the N value is not greater than zero, then evaluatingwhether the N value is less than negative one times the Q value; and ifthe Q value is not less than zero and the N value is greater than zero,then evaluating whether the N value is less than the Q value.
 14. Amethod comprising steps of: making a comparison of Gray code informationwith servo information; and evaluating error in the Gray codeinformation based on the comparison.
 15. The method of claim 14, furthercomprising a step of receiving the Gray code information and the servoinformation from a head interacting with a data medium surface.
 16. Themethod of claim 14, further comprising a step of generating a positionerror signal as a function of the comparison.
 17. The method of claim16, further comprising a step of using the position error signal inpositioning a head relative to tracks on a data medium surface.
 18. Adata storage system, comprising: a data storage medium; a head,controllably positionable relative to the data storage medium; and acontroller, comprising a means for sensing position information from thehead, and a means for compensating for error in the position informationto position the head relative to the data storage medium.
 19. The datastorage system of claim 18, further comprising a means for preventingthe head from writing to the data storage medium when the positioninformation exceeds an error threshold.
 20. The data storage system ofclaim 18, wherein the means for sensing position information from thehead comprises: evaluating whether a Q value comprised in the positioninformation is less than zero and an N value comprised in the positioninformation is greater than zero, and if so, then evaluating whether aGray code value comprised in the position information is even andwhether the N value is greater than negative one times the Q value;evaluating whether the Q value is less than zero and the N value is notgreater than zero, and if so, then evaluating whether the Gray codevalue is even and whether the N value is greater than the Q value;evaluating whether the Q value is not less than zero and the N value isnot greater than zero, and if so, then evaluating whether the Gray codevalue is odd and whether the N value is less than negative one times theQ value; and evaluating whether the Q value is not less than zero andthe N value is greater than zero, and if so, then evaluating whether theGray code value is odd and whether the N value is less than the Q value.